Patients will be given G-CSF by injection under the skin for up to 6 days. Blood and stem cell counts will be measured from a teaspoon of blood drawn each morning. On the morning of the fifth dose, the patient will undergo leukapheresis for collection of stem cells. For this procedure, a large catheter (with a diameter no larger than that of a straw) is placed in an arm vein to allow blood to flow into a cell separator machine. Most children and some adults do not have veins large and strong enough for this tubing, so a large intravenous line called a "central line" is placed into a large vein in the neck or groin. This is done under sedation and with a local anesthetic. While the patient lies on a bed or recliner, whole blood is collected through a catheter in one arm or the central line, the stem cells are separated out by spinning, then the red cells, platelets and plasma are returned through a second catheter in the other arm or a second opening in the central line. The procedure takes about 3 to 5 hours, during which the patient can watch television or videos and have family members at the bedside for company. When the procedure is completed, the patient's participation in the protocol ends.